Although optical fiber communications is a relatively young field, it has reached by now a high level of sophistication. For instance, single mode optical fiber that permits repeater spacings of the order of 30 km and transmission rates of the order of hundreds of megabits/seconds is now available.
Almost all currently used high performance optical fiber is silica-based fiber (comprising more than 50% by weight, typically more than 80%, of SiO.sub.2) typically with a core region having a relatively high refractive index, surrounded by one or more cladding regions of lower refractive index than the core. The indices are raised or lowered, relative to the refractive index of silica, by means of dopants. As is well known, Ge, Al, and P, for instance, raise the refractive index of silica, whereas F and B lower it. Methods for producing optical fiber are well known to those skilled in the art and need no review here. See, for instance, U.S. Pat. No. 4,217,027.
Single mode optical fiber, i.e., optical fiber in which only the fundamental mode (to be designated herein LP.sub.01) of electromagnetic radiation of wavelength equal to the operating wavelength of the fiber propagates with low loss, can have a variety of known index profiles. Typically, the core region and the surrounding cladding region consist of high purity deposited glass material, which is frequently surrounded by a further cladding region consisting of glass derived from a frequently much less pure silica substrate or sleeve tube. The core region typically has a refractive index that is greater than, or at least not less than, that of pure silica, and the deposited cladding region immediately surrounding the core typically has a refractive index that is equal to or less than that of pure silica. The normalized difference between the core refractive index and the refractive index of the cladding region immediately surrounding the core, often designated .DELTA., is typically of the order of 1% or less. For instance, in a commercially available single mode optical fiber, the core has an effective diameter of about 8 .mu.m, and .DELTA. of about 0.40%. In that fiber, the deposited cladding region surrounding the core has an outer radius that is about 6.5 times the radius of the core, and has a normalized refractive index that is about 0.15% less than that of silica. The deposited cladding is surrounded by a second cladding region consisting of material derived from a silica substrate tube, resulting in a fiber outside diameter of about 125 .mu.m. The above described fiber is an example of "depressed" cladding fiber. See, for instance, U.S. Pat. No. 4,447,127, and H. Etzkorn et al, Electronics Letters, Vol. 20 (10), pp. 423-424 (1984). In a "matched" cladding fiber the refractive index of the deposited cladding is substantially equal to that of silica, and therefore, to that of the outer cladding.
Fibers with more complicated index profiles comprising a multiplicity of deposited cladding regions are also known. See, for instance, U.S. Pat. No. 4,435,040, and U.S. patent application Ser. No. 357,053, filed Mar. 11, 1982 by L. G. Cohen et al. The multiple cladding regions known to the prior art typically are designed to affect the transmission characteristics of the optical fiber. In particular, such profiles may be designed to lead to increased bandwidth of the fiber, since the additional cladding layers make it possible to produce fiber having two or more wavelengths of zero dispersion, with low dispersion between the zero-dispersion wavelengths.
A characteristic parameter of optical fiber is the cut-off wavelength .lambda..sub.c, namely, that wavelength above which only the fundamental mode can propagate over significant distances in the fiber. For step index fiber, the theoretical cut-off wavelength is defined by V=2.405, where V=ka(n.sub.c.sup.2 -n.sub.1.sup.2).sup.1/2, with n.sub.c being the core refractive index, and n.sub.1 the refractive index of the deposited cladding surrounding the core. k=2.pi./.lambda., where .lambda. is the wavelength, and a is the radius of the core.
It is to be noted that the index profiles of real fibers typically only approximate the ideal profiles used in theoretical work. However, it is essentially always possible to define an equivalent profile in which all fiber regions have well-defined indices and radii, with the "equivalent" fiber having essentially the same transmission properties as the fiber which it represents. See, for instance, R. J. Black et al, Journal of Lightwave Technology, Vol. LT-2, No. 3, June 1984, pp. 268-276. It is to be understood that the terms "refractive index" and "radius" of a fiber region herein are intended to refer to the equivalent index and equivalent radius of the fiber region, unless otherwise noted.
Although the theoretical cut-off wavelength of a single mode fiber is well defined, in actual fibers the attenuation of higher order modes (most importantly, the first higher order mode designated LP.sub.11) does not become infinite discontinuously, but rather increases over a range of wavelengths. Thus, it is usual practice to define an experimental cut-off wavelength, which typically differs somewhat from the theoretical cut-off wavelength. For instance, the experimental cut-off wavelength can be defined as that wavelength at which the attenuation of LP.sub.11 is 4 dB/m. The term "cut-off wavelength" herein is intended to refer to the experimental cut-off wavelength, unless otherwise noted.
In single mode optical fibers, the fundamental mode should have the lowest possible loss even when the fiber is bent, and the higher order modes should either be absent or have sufficiently high attenuation at the operating wavelength, to avoid significant intermodal dispersion and modal noise. Changes in the index profile of a fiber that strengthen the guiding of the fundamental mode, to thereby reduce the susceptibility of the fiber to macrobending loss (e.g., increasing the V-number by increasing .DELTA.) often strengthen the guiding of higher order modes as well, and thus may not be able to simultaneously further both of the above objectives.
We are disclosing herein a new fiber profile that promotes differential mode attenuation, allowing the fundamental mode guiding to be strengthened relative to that of the secondary modes.